So searching the subject matter did not reveal anything recent like in the past 12 months. So I would like to get your opinion and recommendation on modern new production 6550 tubes for a older generation Mark III amp. My cousin is persuading his co-worker to sell to him his dad's amp. His father passed away some time but his co-worker never tested it. He did indicate there were no tubes except the small one (6AN8). So my cousin will do the typical maintenance but eventually will need to get new power and rectifier tubes. I have a couple spares of JJ 5AR4 to give him, but I have no idea which of the newer made 6550 is recommended for both sound quality and durability. Before anyone post to go with NOS or tested OS from a reliable vendor my cousin is on a tight budget so neither of those options will work at the moment. Besides, I recommended to him for testing he shouldn't use NOS or OS tubes. But he did make a point that he didn't want to go with the cheapest for testing and use better modern tubes when the amp is proven reliable. He wants one purchase to work for him under all situations.